In a typical magnetic array, such as those used for linear motors, alternate magnet segments in the array are magnetically oriented in directions which are 180.degree. rotated from each other about an axis perpendicular to the direction in which the array extends. Such arrays produce magnetic fields which are more or less symmetrically distributed on both sides of the array. However, there are applications where a modified, controlled magnetic field or force pattern may be desired. In particular, since in a typical application only the field on one side of the array is utilized, half of the field is typically wasted. Further, in some applications there may be a need to shield the field on the unused side of the array. Magnetic arrays which provide a selectively controlled field pattern, and more particularly, which are adapted to provide a single-sided field pattern, are therefore desirable.
One type of magnetic array which has been available for some years and which provides a magnetic field which is limited to one side of the array is discussed in several articles by K. Halbach ("Design of Permanent Multipole Magnets With Oriented Rare Earth Cobalt Material", Nuclear Instruments and Methods, Vol. 169, No. 1, pp. 1-10, 1980; and "Application of Permanent Magnets In Accelerators and Electron Storage Rings", Journal of Applied Physics, Vol. 57, No. 8 pp. 3605-3608, 1985). These permanent magnet arrays differ from standard arrays in that each adjacent magnet segment is oriented around an axis perpendicular to the direction in which the array extends by an angle which differs from that of the adjacent segment by a selected angle, for example 45.degree. or 90.degree., which results in magnetic axes which are both array normal and array parallel. As taught by Halbach, the angle of rotation and the direction of rotation are the same for each adjacent pair of magnet segments. An added advantage of these arrays is that they provide a field which is a factor of .sqroot.2 stronger than the field for more conventional magnet arrays with the same volume. These magnet arrays will sometimes be referred to hereinafter as Halbach magnets or Halbach arrays.
However, while Halbach arrays have some interesting properties, they have received little attention and only limited utilization outside of particle physics applications. One potential reason for this is that, while many applications for magnet arrays, such as motors and postioners, require electromagnetic array, Halbach arrays have heretofore only been implemented in permanent magnet form. A need therefore exists for an electromagnetic analog of Halbach arrays which would permit single sided fields to be electromagnetically generated.
Further, it would be desirable if electromagnetic Halbach array could be structured to make the field more uniformly sinusoidal on the strong side of the array and to minimize stray fields on the weak side of the array. Such fields would preferably also be easily switched from one side of the array to the other. More generally, it would be desirable if an electromagnetic array could be structured to provide selected magnetic field patterns as appropriate for a particular application.
Further, Halbach arrays have heretofore been single dimensional arrays. In order to enhance the utility of such arrays, it would be desirable if the principles of such arrays could be adapted to provide three-dimensional field patterns, and more generally, if selected three-dimensional field patterns could be generated by properly structuring magnetic arrays.